The present invention relates generally to a method for reducing four-wave mixing induced degradation in an optical data signal traversing an optical transport network and, more particularly, to a method for reducing four-wave mixing caused by Raman amplifiers operating in long haul, wavelength division multiplexing optical transport networks.
Optical fibers are used to carry optical signals in an optical transport network. However, optical signals may suffer from attenuation (i.e., loss of power) as they travel over long distances. For long fiber spans, the optical signal may be so attenuated that it no longer can be accurately or reliably detected. If the optical signal is to travel further, it must be amplified. Optical amplifiers are employed at different points along the signal path in order to reconstitute the attenuated optical signal, thereby expanding the effective reach of an optical transport network.
Of particular interest, Raman amplifiers may be used to amplify optical signals traversing through long haul optical transport networks. Here, pump sources are used to introduce intense pump light into the transmission fiber to achieve gain via the non-linear Raman effect. In order to achieve wide gain bandwidth, multiple pump laser sources operating at different wavelengths are typically used at a given amplification site.
Four-wave mixing has been a persistent problem in the drive to develop dense wavelength division multiplexing (DWDM) optical transport networks. When multiple wavelengths pass through the same optical fiber, they may interact with each other to generate an additional lightwave. This is known as four-wave mixing. In the context of DWDM optical systems, the additional lightwave will likely interfere with an existing signal channel, thereby causing signal degradation within the system. The efficiency of the FWM product generation depends on many factors, including the fiber dispersion at wavelengths of the interacting light signals, and the polarization orientation of the interacting wavelengths.
Raman amplifiers operating in such DWDM optical transport networks may contribute to the four-wave mixing problem. For instance, the pump lasers associated with a Raman amplifier designed to give gain in the L-band wavelength region (e.g., 1570-1610 nm) may produce four-wave mixing products in the C-band wavelength region (e.g., 1520-1565 nm). The four-wave mixing products in turn cause degradation of the optical data signals residing in the C-band wavelength region. Likewise, Raman amplifiers operating in both L-band and C-band wavelength regions may produce four-wave mixing products in both L-band and C-band wavelength data regions. In the past, this problem did not exist because C and L bands were not combined together in one optical fiber or, if combined, Raman amplifiers have not been used.
Therefore, it is desirable to provide a method for reducing four-wave mixing caused by Raman amplifiers operating in long haul, wavelength division multiplexing optical transport networks.
In accordance with the present invention, an improved method is provided for reducing four-wave mixing induced degradation in an optical data signal traversing an optical transport network that uses Raman amplifiers. The method includes: the presence of at least two Raman pump laser sources operating at different wavelengths that create four wave mixing products in an optical fiber span, and synchronously modulating the laser sources in such a way that at any time, in any section of the fiber span, all of the pump wavelengths required to produce the FWM products are not present at once, thereby preventing the four-wave mixing process. At the same time, pump modulation is performed at such a high rate that crosstalk from the modulation to the optical data signals experiencing the Raman gain is minimized.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.